Conventionally, there is known a processing apparatus in which a substrate to be processed is placed on a substrate stage disposed inside an evacuated processing chamber and in which film-forming processing is performed by CVD method to form a thin film on a surface of the substrate. In this kind of processing apparatus, in case the processing to be performed on the surface of the substrate extends to the outer edge of the substrate so that a film is consequently deposited there, there is the following possibility, i.e., due to the possible peeling off of the film deposited on the outer edge, a bad effect such as particles, contamination and the like will be given to the subsequent processes such as chemical mechanical polishing (CMP) and the like.
As a solution, there is known in patent document 1 an arrangement in which a purge gas assembly is provided to flow a purge gas from a side of the substrate stage, to thereby prevent the processing to be performed on the surface of the substrate from affecting the outer edge of the substrate. In this arrangement, a shoulder portion is disposed on an outer circumferential portion of the substrate stage in a manner to offset downward below a substrate mounting surface which lies on an upper end of the substrate stage. A purge ring is disposed on the shoulder portion so as to enclose a circumferential surface of a stepped portion between the substrate mounting surface and the shoulder portion. An annular gas ejection passage to eject the purge gas is defined between the circumferential surface of the stepped portion and an inner circumferential surface of the purge ring. According to this configuration, the outer edge of the substrate is covered by the purge gas to be ejected from the gas ejection passage. As a result, the processing by the processing gas will not reach the outer edge of the substrate.
Unless the purge gas is ejected uniformly out of the gas ejection passage over the entire circumference thereof, there is a possibility that the film distribution on the surface of the substrate becomes abnormal. As a solution, in the arrangement described in the patent document 1, there is formed an annular recessed groove on an upper surface of the shoulder portion of the substrate stage. Purge gas inlets are provided at a plurality of positions in the circumferential direction on the bottom surface of the recessed groove. The purge gas is thus distributed in the circumferential direction inside the recessed groove before being supplied to the gas ejection passage.
However, due to the dimensional restriction in the shoulder portion, the cross sectional area of the recessed groove cannot be made so large, so that the distribution of the purge gas in the circumferential direction cannot be made uniform by the recessed groove alone. As a solution, in the arrangement described in the patent document 1, a coiled member is disposed within the recessed groove. By passing the purge gas through this member, the distribution in the circumferential direction can be made uniform.
In addition, in case the film-forming processing is performed by the CVD method, it is so arranged that the substrate is heated by the heat transmission from the substrate stage by means of a heating means for the substrate stage so that the film-forming processing on the surface of the substrate is accelerated. In this case, if the purge ring is heated by the heat transmission from the substrate stage, film is likely to be deposited on the upper surface of the purge ring. Therefore, in the arrangement descried in the patent document 1, the purge ring is constituted by a plurality of upper and lower ring members. Groove portions which serve to reduce the area of contact are formed between the ring members. In this manner, the heat to be transmitted from the substrate stage to the ring member on the upper side through the ring member on the lower side is restricted so that the film is hardly deposited on the upper surface of the uppermost ring member.
Patent document 1: JP-A-2001-297991